Equipment safety and related aspects

P O Risman has a very long career in the international standardisation work on microwave oven safety and performance. He was Chairman of the international IEC (International Electrotechnical Commission) committees for microwave oven safety testing and performance measurements 1977–97 and a member until 2014. For many years, he has also been the sole Swedish representative in Technical Committee 27 (the IEC committee for industrial electroheat) and is now in effect leading the modernisation work of the main safety standard. He is also since many years very active in the modernisation of the particular standards for microwave equipment and installations, and was appointed as global convenor for the international team dealing with this, in early 2010. In May 2009 he was appointed to become the global coordinator for energy efficiency measurement standards for industrial electroheat, rewsulting in a Techn ical Specification which is now included in the main safety standard for industrial eledctroheating and electromagnetic processing. 

The first international comprehensive safety standard for microwave tunnel ovens was published in February 2006. P O was a very active member in the working group which developed this global IEC standard, contained in 60335-90 Ed.3. –  Several terms and definitions for tunnel ovens are included. – P O also authored an informative annex dealing with different kinds of leakage, the various kinds of hazards that may be created, and the proper way to handle them from a safety assessment perspective. Exposure time issues are also dealt with and have a direct impact on the general protection against microwaves dealt with by radiation protection agencies. The annex can be downloaded here (rtf format). – A modified leakage measurement probe is required  for correct measurements of microwaves (closer than the earlier stipulated 50 mm away from any part of the equipment) that may be hazardous in microwave tunnel ovens operated by unqualified personnel.

From 2012, P O has been instrumental and ther most active of the two Working Group co-convenors in the development of two normative IEC Technical Specifications: Magnetic nearfield hazards 1 Hz to 6 MHz, and Touch current hazards from 1 kHz to 6 MHz.These specifications will be the first in their kinds, in spite of  hazards by internal electric fields in bodyparts being one of the main tasks of the IEC ever since it was created in 1906.

As to the magnetic nearfield hazards, the most basic and valid relationship between an inducing magnetic flux intensity B in the small region where the induced electric field strength E is highest is expressed as E = C·f·B  in the SI system, where C is a coupling value in meter and f the frequency in hertz. It follows from the definition of the coupling value C that it is applicable if the magnetic field is not affected by the presence of the object, i.e. if the object is non-magnetic and has an electric conductivity σ which is sufficiently low for the E field not to be affected by it. Furthermore, humans are considered entirely non-magnetic so the in situ B = µ0H. It is also to be noted that C is in principle frequency independent up to about 6 MHz, since the magnetic field is then not influenced by the presence of the bodypart. – The images on the startpage show a scenario of a hand with spread-out fingers, 6 mm above an induction coil. The Ccoil values (i.e. with the B field at the coil centre rather than at the hand locations as reference) were obtained by advanced numerical (FDTD) modelling and it was found that the maximal value is about 0,33 m, then at 5 mm above the hand underside. The Ccoil value at the inner ends of the fingers was maximally about 0,40 m, then 8 mm above the hand underside. It should be noted that the existing Directive 2013/35/EU on the minimum heath and safety requirements regarding the exposure of workers corresponds to C = 0,77 m, with the reference being the B value at the hand location; this is significantly higher than that used for the determination of the Ccoil value. Hence, it is quite obvious that the EU Directive is too onerous for the situation of a hand near a coil

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For those interested in graphical symbols of microwave components and the corresponding Swedish terms, a scanned multipage TIFdocument from 1961 is also uploaded.

P O Risman regularly assists client companies in CE marking and verifications of microwave heating equipment and installations. He also evaluates equipment/installation safety and performance – whether for a complaining user or an insurance company after a fire accident. As a part of the worker safety assurance and preparedness for possible microwave fear, he regularly holds half- or full-day employee courses in conjunction with the start-up of large installations, and also at laboratories dealing with microwave equipment, and in similar situations.

Three documents on microwave safety can be obtained here. They are all in Swedish. For English versions, contact us.
        •   An overhead illustration, on the need for information before possible exposure
        •   Answers to some common questions.
        •   Example of a personnel safety instruction sheet.